Infectivity and pathogenicity of Bordetella bronchiseptica to rabbits were investigated by intranasal inoculation of the organism to suckling and young animals. Results are summarized as follows. 1. By inoculation of 5 X 10(4) and 10(6) organisms, the infection developed in 60% and 100% of young rabbits, respectively. 2. In young rabbits, the growth of the organism was evident in the trachea within 5 days after inoculation and in the lung on the 10th day. The organism began to disappear from the lung and trachea of some animals from the 40th day after inoculation, but remained in the external nares and nasal cavity of all animals even on the 90th day. Neither clinical sign nor pneumonic lesion was observed in any stages of the infection. 3. Pneumonic lesions and serous nasal secretion were found in all suckling rabbits by inoculation of 5 X 10(6) organisms, but no fatal case was detected.